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Sheldrickite Abstracted by Malcolm Back
The specimens were collected in the Poudrette quarry from a marble xenolith in nepheline syenite, close to a large unit of hornfels. Sheldrickite is intimately associated with shortite, the crystals being found in a cavity between shortite crystals and as flakes in thin seams between crystals of shortite. Other associated species include pectolite, microcline, polylithionite, arfvedsonite, aegirine, calcite, fluorite, and minor molybdenite, leucosphenite, thenardite, thermonatrite, sphalerite, galena, schairerite and kogarkoite. The mineral assemblage is thought to be a late-stage hydrothermal infilling. Sheldrickite has two distinct habits: (1) it occurs as a 1 x 1 x 2 mm aggregate of blocky twinned crystals with individuals up to 0.1 mm, and (2) more commonly as radiating thin white silky flakes to fibrous masses up to 2 mm wide. The mineral is colorless to white with a vitreous lustre. It is brittle with an uneven fracture, and has a good {001} parting. Sheldrickite is relatively soft (Mohs hardness 3) and does not fluoresce. It is uniaxial positive, _ = 1.583(2) and e = 1.563(4)(for l = 589 nm) and nonpleochroic. Sheldrickite is trigonal, space group P32 , a 6.718(3)Å, c 15.050(4) Å, V 588.3(3) Å3, and Z = 3.
Source:Grice, J. D., Gault, R. A., and Van Velthuizen, J. 1997. Sheldrickite, A New Sodium-Calcium-Fluorocarbonate Mineral Species from Mont Saint-Hilaire, Québec. Can. Min. 35:181-187.
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